Ferromagnetic resonance magnetometer

ABSTRACT

A magnetometer having a thin-film sensor positioned near or at the end of a transmission line and biased near its ferromagnetic resonance. An electromagnetic signal is applied to the thin-film sensor. The reflected portion of this signal is amplitude modulated by the influence of an external magnetic field on the thin-film sensor, and then detected to provide a measure of the external magnetic field.

United States Patent Irons et a1.

FERROMAGNETIC RESONANCE [15] 3,693,072 [451 Sept. 19,1972

3,320,554 5/1967 Wieder ..324/0.5 UX MAGNETOMETER OTHER PUBLICATIONS[72] Inventors: Henry R. Irons, Adelphl', Leonard J.

s h Silver spring, both f P. hgiiordon SVanable CouplirliagReflfegtionlCavity t1 C. t [73] Assignee: The United States of Americaas ggzsn firz f W o 1 ns t "Presented by the Seem? of the R. M. Rogers &R. H. Kantor- Frequency Shift Mag- Navy netometer Rev. of Sci. Instr.32(11) Nov. 1961 22 Filed: Aug. 25, 1967 PP- 1230-1234 [21] Appl'663,454 Primary Examiner-Michael]. Lynch Attorney-R. S. Sciascia, J. A.Cooke and S. Sheinbein [52] US. Cl. ..324/0.S R 51 Im. Cl. ..G0lr 33/08ABSTRACT Fleld 0f A magnetometer having a Sensor po i 56 R i Ci ed nearor at the end of .a transmission line and biased 1 e memes near itsferromagnetic resonance. An electromagnetic UNITED STATES PATENTS signalis applied t0 the thin film sensor. The reflected portion of this signal18 amplitude modulated by the m- 3,l00,866 8/1963 Zimmermam. ..324/0.5fluence of an external magnetic field on the thi fi] 3,158,802 11/1964Jung ..324/0.5 Sensor and the detected to provide a measure of the3,227,944 1/1966 Hasty ..324/0.5 external magnetic u 3,239,754 3/1966Odom ..324/0.5 3,441,837 4/1969 Desormiere ..324/0.5 7 Claims, 6 DrawingFigures 27 BIASING MAGNET l4 FILM SENSOR /0 a 20 22 O OSCILLATORCIRCULATOR DETECTOR FILTER AMPLIFIER SCOPE N UE SHEET 1 OF 2 mmOow mEEEs? mmljazq PATENTEI] SEP I 9 I972 moBwBm no: 20228 mowzww 2:. t .9

523: $5 5m kw INVENTORS Henry R.. Irons ATTOR Y Leona rd ..J. Schwee BY8 O FERROMAGNETIC RESONANCE MAGNETOMETER BACKGROUND OF THE INVENTION hasbeen a general practice to operate near a resonance l0 determined bythin-film inductors and capacitors. The prior art magnetometers also usehigh Q circuits and low carrier frequencies which tends to limit theresponse time of the magnetometer. The use of such high Q circuits andlow carrier frequencies also requires that the electronic components ofthe magnetometer be locatedclose to the thin-film sensor, thus makingfield use difficult in some applications.

While prior art magnetoresistive magnetometers have been satisfactoryfor high frequency applications, they require electrical connections tobe made to the thin-film sensor. Bridge techniques were required tomodulate the carrier signal thus resulting in much waste of the carrierlevel and hence a lower index of modulation for the same carrier signalstrength.

SUMMARY OF THE INVENTION Accordingly, one object of the presentinvention is the provision of a new and improved ferromagnetic resonancemagnetometer.

Another object of the present invention is the provision of a new andimproved magnetometer for detecting external magnetic fields byutilizing the impedance of a thin-film sensoritself.

A further object of this invention is the provision of a new andimproved magnetometer with fast response time and high sensitivity.

Still another object of the present invention is the provision of a newand improved magnetometer which is particularly adaptable for fielduse.

Another still further object of the instant invention is the provisionof a new and improved magnetometer for readily measuring magnetic fieldsof frequencies between do and 100 MHz and field strengths between and 350e.

Another still further object of the subject invention is the provisionof a new and improved magnetometer wherein a high index of modulationcan be easily obtained.

Briefly, in accordance with this invention the foregoing and otherobjects are attained by placing a thin-film sensor in an externalmagnetic field to be measured and applying an electromagnetic signal tothe thin-film sensor which is amplitude modulated by the externalmagnetic field and is then measured by detecting the amount of voltagereflected from the thin-film sensor.

BRIEF DESCRIPTION OF THE DRAWING A more complete appreciation of theinvention will be readily appreciated as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with accompanying drawings wherein:

FIG. 1 is a block diagram showing one preferred embodiment of aferromagnetic resonance magnetometer in accordance with this invention;

FIG. 2 is a schematic illustration of another embodiment of'aferromagnetic resonance magnetometer in accordance with this invention;

FIGS. 3A, 3B and 3C illustrate three techniques for matching a thin-filmsensor to a transmission line; and

FIG. 4 illustrates a typical resonance curve for the voltage reflectedfrom a thin-film sensor as a function of magnetic field.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawingswherein like reference numerals designate identical, or correspondingparts throughout the several views, and more particularly to FIG. 1thereof wherein a magnetometer in accordance with the present inventionis shown as consisting of a source of electromagnetic carrier signal ofa frequency of 10 MHz to 2 GHz such as an oscillator 10 which supplies acarrier signal through a unidirectional flow circulator 12 to athin-film sensor 14 by way of a transmission line 16. By way of example,the thin-film sensor 14 may consist of thin-films of percent nickel and20 percent iron with zero magneto-:striction. It will be readilyapparent however, to those of ordinary skill in the art that thin-filmsof other alloys and percentage combination may also be used.Transmission line 16, by way of example, may be a 50 ohm cable. The highfrequency electromagnetic carrier signal allows the use of a single longtransmission line which makes the magnetometer of the present inventionparticularly adaptable for field use. When the thin-film sensor isplaced in an external magnetic field, shown as H, the reflected carriersignali l0 will be amplitude modulated by the influence of the externalmagnetic field on the sensor 14. The modulated carrier wave is thendetected by a detector 18 which for example may be a full waverectifier, and filtered by a low pass filter 20; The filter 20 is usedto smooth the output of detector 18 so that the ripple will not saturatean amplifier 22 which may follow. After the modulated carrier wave isdetected, filtered and amplified it is applied to an oscilloscope 24.The vertical deflection of the oscilloscope 24 is proportional to thevoltage amplitude of the wave reflected from the impedance load providedby the thin-film sensor 14 at the end of the transmission line 16. Thisreflected wave is in turn a measure of the external magnetic field H,into which the thin-film was inserted.

It should be understood that modifications may be made to the aforedescribed embodiment. For example, as shown in FIG. 2 a conventionalhigh-pass filter and common mode rejector 21 maybe inserted intotransmission line 16 to eliminateany current induced by the electricalfields associated with the dynamic magnetic fields present. Additionallya directional coupler may be substituted for circulator 12.

In actual operation, the thin-film sensor 14 is coupled to thetransmission line 16, in a manner more fully explained hereinafter inreference to FIGS. 3A, 3B and 3C such that at the peak of the resonancecurve of the thin-film i.e., the point to in FIG. 4, the thin-filmsensor provides a matched impedance load for the transmission line 16and there is no reflected wave. At fields far from the resonance fieldthe thin-film provides only a small reactive impedance and the incidentwave is almost totally reflected. With a fixed carrier signal thethin-film is then biased at point P by a ceramic permanent magnet 27(shown in FIGS. 1 and 2) with an initial permeability near unity andmagnetic field strength I-l so that with no external magnetic fieldpresent, the voltage amplitude of the reflected wave is approximatelyone-half the incident wave. By choosing such a biasing point thethin-film sensor is allowed to operate in a reasonably linear range.This linear operating range is shown by the solid linear line in FIG. 4.This linear relationship between the voltage amplitude of the reflectedwave and the magnetic field allows for an accurate measurement of theexternal magnetic field to be readily obtained.

The biasing magnets used are economical, weighing but a few ounces. Forexample, for carrier frequencies between 300 MHz and 1.3 GI-lz, smallbiasing fields between 1 and 60 e need only be provided. It should beunderstood, however, that other conventional magnetic biasing methodsmay be used. When the thin-film sensor is placed in an external magneticfield H, to be detected, it will add to or subtract from the biasingfield increasing or decreasing the amplitude of the reflected wavedepending upon the direction of the field. The voltage amplitude of thereflected wave thus gives a direct measurement of the external magneticfield H When sufficiently high carrier frequencies are used, thethin-film magnetometer may have a response time of less than 4 nsec. Inaddition, the magnetometer of the present invention responds linearlywith a large dynamic range. In particular, measurement of magneticfields of frequencies between d.c. and 100 MHz and field strengthbetween Oe and 35 Oe may be readily realized. It should be understoodhowever that by slight modification ranges larger than those statedabove can be obtained. For example, the sensitivity of the magnetometeris determined by the line width of the thinfilm used, the band widthrequired, and the transmission line used. For narrowerv band width,smaller fields can be measured. For larger line widths greater fieldsmay be measured. It is further to be understood that any desiredcombination of the above changes can be readily made.

Considering the fact that the information about the external magneticfield H, is gained through amplitude modulation of the carrier signal,it is desirable to have as high an index of modulation as possible inorder to minimize the noise. When the thin-film sensor is matched to thetransmission line at resonance, the reflected carrier frequency can beamplitude modulated up to 70 percent by the thin-film sensor with theresponse linear to within 3 percent.

Three methods of matching the impedance provided by the thin-film atresonance to the transmission line are shown in FIGS. 3A, 3B and 3C.

:ln the first method shown in FIG. 3A, a magnetic film 1,500 A thick isdeposited on a one micron thick layer of silicon monoxide which has beenpreviously deposited on a 1,000 A of layer of sodium chloride. All thelayers are made by vacuum deposition onto a heated glass substrate. Whenwater is applied to the completed film, the salt is desolved, and thesilicon monoxide magnetic thin-film layer 13 is removed from the glasssubstrate and wound around a onemm copper wire 23. The wire 23 isconnected to the end of the transmission line 16.

A second method is shown in FIG. 3B. Thin-films 13 on thin glasssubstrates about 0.1 mm are stacked and placed in a shorted loop at theend of transmission line 16. The number of films required variesdepending on the desired thickness, line width, and frequency. Thedimensions of the thin-films used vary depending on frequency and arealways shorter than one eighth the wavelength. The width normally usedis about 0.5 cm.

The third method shown in FIG. 3C utilizes a quarter wave matchingsection to match thin-films 13 on glass substrates placed between thecenter conductor 26 and ground planes 28 of strip lines just before ashort.

What is claimed and desired to be secured by Letters Patent of theUnited States is: v

1. A ferromagnetic resonance magnetometer comprising:

a thin-film sensor formed of ferromagnetic material,

means biasing said thin-film sensor near its ferromagnetic resonance bya ceramic permanent magnet with an initial permeability near unity,means generating a fixed period, extremely high frequencyelectromagnetic signal which is modulated when said sensor defects anexternal magnetic field,

transmission line means coupled to said generating means and to saidthin-film sensor and impedance matched to said thin-film sensor forapplying said generated electromagnetic signal to said sensor and fortransmitting the reflected signal therefrom,

means responsive to the amplitude of said reflected signal as it varieswhen said ferromagnetic resonance is shifted by said external magneticfield, and

means coupled to said sensor and said reflected signal responsive meansfor effecting the unidirectional signal flow from said sensor to saidreflected signal responsive means.

2. A ferromagnetic resonance magnetometer as i claim 1 wherein saidunidirectional signal flow means is a circulator.

3. A ferromagnetic resonance magnetometer as in claim 1 wherein saidresponsive means comprises a detector circuit for detecting the voltageamplitude of said reflected signal.

4. A ferromagnetic resonance magnetometer as in claim 3 wherein saidresponsive means further comprises a low pass filter for smoothing theoutput of said detector circuit, an amplifier for amplifying saidsmoothed output, and an oscilloscope for directly determining themagnitude of said magnetic field.

5. A ferromagnetic resonance magnetometer as in claim 1 wherein a highpass filter and a common mode rejector are inserted into saidtransmission line means for eliminating induced currents.

6. A ferromagnetic resonance magnetometer as in claim 1 wherein saidthin-film sensor comprises a thinfilm wound around a fine wire, saidwire coupled to said transmission line, whereby said transmission lineis impedance matched to said thin-film sensor.

7. A ferromagnetic resonance magnetometer as in claim 1 wherein saidthin-film sensor comprises a stack of thin-films, each film mounted on athin substrate, said thinfilms coupled to an end of said transmissionline in a shorted loop whereby said transmission line is impedancematched to said thin-film sensor.

1. A ferromagnetic resonance magnetometer comprising: a thin-film sensorformed of ferromagnetic material, means biasing said thin-film sensornear its ferromagnetic resonance by a ceramic permanent magnet with aninitial permeability near unity, means generating a fixed period,extremely high frequency electromagnetic signal which is modulated whensaid sensor defects an external magnetic field, transmission line meanscoupled to said generating means and to said thin-film sensor andimpedance matched to said thin-film sensor for applying said generatedelectromagnetic signal to said sensor and for transmitting the reflectedsignal therefrom, means responsive to the amplitude of said reflectedsignal as it varies when said ferromagnetic resonance is shifted by saidexternal magnetic field, and means coupled to said sensor and saidreflected signal responsive means for effecting the unidirectionalsignal flow from said sensor to said reflected signal responsive means.2. A ferromagnetic resonance magnetometer as in claim 1 wherein saidunidirectional signal flow means is a circulator.
 3. A ferromagneticresonance magnetometer as in claim 1 wherein said responsive meanscomprises a detector circuit for detecting the voltage amplitude of saidreflected signal.
 4. A ferromagnetic resonance magnetometer as in claim3 wherein said responsive means further comprises a low pass filter forsmoothing the output of said detector circuit, an amplifier foramplifying said smoothed output, and an oscilloscope for directlydetermining the magnitude of said magnetic field.
 5. A ferromagneticresonance magnetometer as in claim 1 wherein a high pass filter and acommon mode rejector are inserted into said transmission line means foreliminating induced currents.
 6. A ferromagnetic resonance magnetometeras in claim 1 wherein said thin-film sensor comprises a thin-film woundaround a fine wire, said wire coupled to said transmission line, wherebysaid transmission line is impedance matched to said thin-film sensor. 7.A ferromagnetic resonance magnetometer as in claim 1 wherein saidthin-film sensor comprises a stack of thin-films, each film mounted on athin substrate, said thin-films coupled to an end of said transmissionline in a shorted loop whereby said transmission line is impedancematched to said thin-film sensor.